linux-kernel - Re: [PATCH 1/2] sched/fair: move cpufreq hook to update_cfs_rq_load

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Message-ID: <570E8A8F.2030109@linaro.org>
Date:	Wed, 13 Apr 2016 11:06:07 -0700
From:	Steve Muckle <steve.muckle@...aro.org>
To:	"Rafael J. Wysocki" <rafael@...nel.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Ingo Molnar <mingo@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Juri Lelli <Juri.Lelli@....com>,
	Patrick Bellasi <patrick.bellasi@....com>,
	Michael Turquette <mturquette@...libre.com>
Subject: Re: [PATCH 1/2] sched/fair: move cpufreq hook to
 update_cfs_rq_load_avg()

On 04/13/2016 09:07 AM, Rafael J. Wysocki wrote:
>>>>> If you want to do remote updates, I guess that will require an
>>>>> irq_work to run the update on the target CPU, but then you'll probably
>>>>> want to neglect the rate limit on it as well, so it looks like a
>>>>> "need_update" flag in struct update_util_data will be useful for that.

Have you added rate limiting at the hook level that I missed? I thought
it was just inside schedutil.

>>>>
>>>> Why is it required to run the update on the target CPU?
>>>
>>> The fast switching and intel_pstate are the main reason.
>>>
>>> They both have to write to registers of the target CPU and the code to
>>> do that needs to run on that CPU.

Ok thanks, I'll take another look at this.

I was thinking it might be nice to be able to push the decision on
whether to send the IPI in to the governor/hook client. For example in
the schedutil case, you don't need to IPI if sugov_should_update_freq()
= false (outside the slight chance it might be true when it runs on the
target). Beyond that perhaps for policy reasons it's desired to not send
the IPI if next_freq <= cur_freq, etc.

>> And these two seem to be the only interesting cases for you, because
>> if you need to work for the worker thread to schedule to eventually
> 
> s/work/wait/ (sorry)
> 
>> change the CPU frequency for you, that will defeat the whole purpose
>> here.

I was hoping to submit at some point a patch to change the context for
slow path frequency changes to RT or DL context, so this would benefit
that case as well.

thanks,
steve